Many situations arise in which it is necessary or desirable to deliver a fluid through an outer conduit via an inner conduit positioned in the outer conduit. Such situations commonly arise in the drilling or completion of boreholes or in the production of fluids from boreholes.
In particular, it is customary to line boreholes with a liner conduit in order to provide structural integrity to the borehole and to facilitate the carrying out of subsequent operations in the borehole. Often these liner conduits are comprised of a casing string, a liner or both.
Typically liner conduits are positioned and then permanently fixed in the borehole using cement or some other bonding material which is placed in the annular space between the liner conduit and the borehole wall. This bonding material is usually delivered to the bottom of the liner conduit and then forced under pressure into the annular space between the liner conduit and the borehole wall where it is permitted to set and form a bond between the liner conduit and the borehole.
The challenge in these "bonding" or "cementing" operations is to deliver the bonding material to the annular space surrounding the liner conduit without permitting the bonding material to move significantly into the interior of the liner conduit itself.
As a result, cementing operations in boreholes are typically carried out using a cementing apparatus which is comprised of an inner conduit which is temporarily placed inside the liner conduit. Fluids such as flushing agents and the bonding material are then passed down the borehole through the inner conduit and released adjacent to the lower end of the liner conduit. In order to prevent these fluids from circulating back up through the liner conduit, it is customary to include a bushing device as part of the cementing apparatus to inhibit the passage of fluids through the interior of the liner conduit after the fluids are released from the inner conduit by providing a full or partial seal between the liner conduit and the inner conduit.
Several challenges are faced in the design and operation of these bushing devices. First, the bushing is exposed to fluid pressure during cementing operations from the fluid which is passed down through the inner conduit and released into the liner conduit. This fluid pressure is significant, tends to push the bushing and/or the inner conduit upwards through the interior of the liner conduit and must be counteracted in some manner, usually by anchoring the bushing in the liner conduit.
Second, since the bushing is intended to provide a seal between the liner conduit and the inner conduit, it occupies a significant amount of space inside the liner conduit. It is therefore desirable that the bushing be removable from the liner conduit in whole or in part after cementing operations are completed in order to minimize the amount of permanent obstruction in the liner conduit caused by the bushing.
Cementing operations may be carried out independently of the task of positioning the liner conduit in the borehole. In situations, however where the liner conduit is an extension of a previously installed liner conduit, cementing operations are often carried out in conjunction with the task of positioning and hanging the liner conduit in order to avoid multiple trips into the borehole and in order to save costs.
As a result, cementing apparatus are often included as a component of liner hanging apparatus, and must therefore often be designed to be compatible with the operational requirements of such liner hanging apparatus. Specifically, the liner hanging apparatus typically includes the inner conduit, which must be manipulated rotationally and/or axially during hanging of the liner. This requirement adds a further challenge to the design of the bushing in that the bushing must accommodate this manipulation, leaving the inner conduit potentially vulnerable during cementing operations to forces exerted by the fluid which tend to push the inner conduit upward in the liner conduit.
The prior art describes several attempts at designing cement bushings or packers for use in association with liner hanging apparatus which can be anchored in the liner conduit and then removed from the liner after use along with the liner hanging apparatus.
U.S. Pat. No. B1 3,920,075 (Braddick et al) and U.S. Pat. No. B1 4,281,711 (Braddick et al) both describe a method and apparatus for bonding a liner conduit in a wellbore which include or utilize a retrievable seal bushing. The seal bushing is releasably locked to the liner conduit and the liner conduit is releasably engaged with an inner conduit described as a well string. The liner conduit and the well string are lowered into the wellbore and the liner conduit is positioned at the desired location in the wellbore. The liner conduit is then disengaged from the well string by rotation of the well string relative to the liner conduit. Bonding operations are carried out by passing a bonding agent through the well string, after which the seal bushing is unlocked from the liner conduit by longitudinal movement of the well string relative to the bushing. Finally, the well string and the seal bushing are retrieved from the wellbore.
Braddick et al rely upon a relatively rugged collar on the well string both to support the well string against upward axial forces exerted on the well string by the fluid and to provide a shoulder which engages the seal bushing upon upward longitudinal movement of the well string to unlock the seal bushing by shearing a shear pin associated with the seal bushing. Braddick et al do not otherwise address the upward axial force which is exerted on the well string, other than to acknowledge that this force is undesirable due to the effect it has on the well string. In particular, Braddick et al do not disclose, teach or suggest how this upward axial force might be reduced or minimized.
U.S. Pat. No. 5,857,524 (Harris et al) describes a liner hanging, sealing and cementing tool including two different embodiments of cement bushing. It is expressed in Harris et al that a main purpose of the cement bushing is to hold the inner conduit (described as a drill pipe) in the liner conduit against the high upward force occurring during pumping of cement through the drill pipe. This purpose is addressed in Harris et al by providing an arrangement of slips or pistons associated with the drill pipe which are actuated by fluid pressure to engage the inner surface of the liner conduit and thus hold the drill pipe in place in the liner conduit. Harris et al does not disclose, teach or suggest how this upward force might be reduced or minimized.
Texas Iron Works, Inc. of Houston, Tex, manufactures a retrievable pack-off bushing based upon the Braddick et al patents discussed above. In advertising material relating to this pack-off bushing it is mentioned that the effects of the upward axial force acting on the inner conduit and the bushing can be reduced by minimizing the cross-sectional area of that portion of the inner conduit which is exposed to fluid pressure during cementing operations.
None of the prior art references outlined above fully address the effects of the upward axial force that is typically exerted on the inner conduit during cementing operations.
There therefore remains a need for a cement bushing which is designed to reduce the magnitude of the upward axial force exerted on the inner conduit during cementing operations. There is also a need for such a cement bushing that may be used in conjunction with liner hanging apparatus. Finally, there is a need for such a cement bushing which is retrievable from the liner conduit after cementing operations are complete.